FIELD OF THE INVENTION
The present invention relates to a stress determination method in heat
exchanger(s) provided as part of steam generators (or boilers). The invention
utilises optical techniques and fiber optic sensors in realising this measurement.
More particularly, the invention relates to a method for determination of
developed stress in the pressure components and heat exchangers in a steam
generator.
BACKGROUND OF THE INVENTION
Steam generators are employed world-wide by thermal power plants, refineries
and many other industries to produce steam for their operation. Many of the
boilers are designed for firing fuel oil and / or fuel gas and / or coal or other
fuels. After ensuring proper conditions for firing the fuel, the fuel is admitted into
the furnace of the boiler. The fuel gets lighted-up by igniters, the flame gets
established and hot flue gas passes through the furnace and outside of various
pressure parts and heat exchangers located inside boiler passes. The heat
transfer takes place through radiation and / or convection. The various pressure
parts and heat exchangers like superheater or reheater tubes are cooled by
steam passing through the tubes.
The metals of the pressure components and heat exchangers are exposed to
high temperature and high pressure steam passing inside, and subjected to high
stress conditions. High grades of alloy steel are used to withstand the physical
stress imposed by elevated temperatures and pressures.
Monitoring the condition of heat exchanging surface is crucial in the steam
generator plant operating at a relatively high stress and pressure. In particular,

the critical parameters are thermal gradient across the heat exchanging surface
and associated stress developed due to the thermal gradient.
Conventional stress determination is done in a system by measurement of strain
the system is subjected to - as a result of the stress developed in it. A resistance
strain gauge is used for this purpose. The resistance of a strain gauge increases
when put under tensile stress due to the intrinsic change in the material while
under load i.e., the increase in length and the reduction of cross-section of the
gauge element. The gauge is bonded to the surface of interest. The resistance
change in the gauge is related to the average strain under the gauge length.
One of the major disadvantages of prior art system is the difference in thermal
expansion properties of common metals vis-a-vis the strain gauge material. The
variability of intrinsic resistance of the strain gauge material with the
development of stress leads to loss of elastic properties in the strain gauge
element under operating conditions of the steam generator, is another
disadvantages that generally forbids the use of these instruments in heat
exchanging apparatus of a steam generator system.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention is to propose a method for
determination of developed stress in the pressure components and heat
exchangers in a steam generator, which adapts an optical sensing means.
Another object of the invention is to propose a method for determination of
developed stress in the pressure components and heat exchangers in a steam
generator, which determines the developed stress based on the related change
in a physical property of the optical sensing means.

A still another object of the invention is to propose a method for determination
of developed stress in the pressure components and heat exchangers in a steam
generator, which utilizes an array of multi point sensors arranged in a single
optical means.
A further object of the invention is to propose a method for determination of
developed stress in the pressure components and heat exchangers in a steam
generator, which considers the interference signal produced by the measurand
induced change in the optical sensing means.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 shows the principle of Fiber optic based Stress developed sensor
DETAILED DESCRIPTION OF THE INVENTION W.R.T NUMBERED
PARTS
According to the invention, stress developed in pressure parts and heat
exchangers of a steam generator is determined by using an optical sensing
means (1). The optical sensing means (1) in the form of an array is deployed
within the steam generator (2). The embedded optical sensing means (1)
receives the light originating from a light source when launched into the optical
sensor via a fiber optic cable (3). The incident light undergoes a reflection within
the sensing means (1) at a reflector located at the far end of the sensing means
(1). The reflected light interacts with incident light to produce an interference
signal pattern at a junction between a light guide and the reflector within the
optical sensing means. As the sensing means undergoes geometric deformation
due to change in process parameters in the steam generator, the extent of

change in the interference signal varies relatively. As the light travels through the
optical sensing means, the interference signal undergoes a relative modulation. A
signal interrogating means (4) is deployed at the end of the optical array (1) to
determine the measurand value at the sensor points as a function of the
modulation in the interference signal.
A signal processor (5) provides a measurand profile of the pressure components
and the heat exchangers in the steam generator from measured values of the
point stress developed on the sensor elements.

WE CLAIMS
1. A method to determine stress developed in heat exchanging structure of
a steam generator, comprising the step:
- providing an optical sensor means being deployed in the generator;
- launching light from a light source to be incident on a light reflector via
an optical cable;
- allowing interaction of the reflected light with the incident light to
produce an interference pattern at a junction between a light guide
and the reflector within the optical sensing means;
- providing a signal interrogating means to determine modulated values
of the interference signal due to change in process parameters
corresponding to geometric deformation of the optical sensing means;
and
- providing a signal processor to output a measured profile of stress of
the pressure components and heat exchangers based on the inputted
values of the modulated interference signal.
2. The method as claimed in claim 1, wherein the components comprise
evaporator tubes, superheater tubes, reheater tubes, separator, steam
headers and water headers.

The invention relates to a method to determine stress developed in heat
exchanging structure of a steam generator, comprising the step providing an
optical sensor means being deployed in the generator; launching light from a
light source to be incident on a light reflector via an optical cable; allowing
interaction of the reflected light with the incident light to produce an interference
pattern at a junction between a light guide and the reflector within the optical
sensing means; providing a signal interrogating means to determine modulated
values of the interference signal due to change in process parameters
corresponding to geometric deformation of the optical sensing means; and
providing a signal processor to output a measured profile of stress of the
pressure components and heat exchangers based on the inputted measure
values of the modulated interference signal.